The oxidation of cyclohexane and subsequent separation of a mixture of cyclohexanone and cyclohexanol from unreacted cyclohexane is a key step in the manufacture of both adipic acid and caprolactam. The primary industrial use of caprolactam is as a monomer in the production of nylon-6. Adipic acid is a monomer used in the production of nylon-6,6 amongst other applications.
Conventionally, the oxidation of cyclohexane is carried out at a relatively low conversion of less than 10%. The primary oxidation products of cyclohexane are cyclohexane hydroperoxide, cyclohexanol and cyclohexanone. In a typical commercial cyclohexane oxidation process cyclohexyl hydroperoxide is then decomposed to cyclohexanol and cyclohexanone, either in the reactor or in a separate unit operation. This process as a whole can be described for the purposes of the present invention as the cyclohexane oxidation process. The desired final oxidation product after decomposition of cyclohexyl hydroperoxide is a mixture of primarily cyclohexanone and cyclohexanol. The mixture must then be separated from the unreacted cyclohexane whereby the unreacted cyclohexane can then typically be recycled to the oxidation reaction. This
separation is commercially carried out by distillation, and since the great majority of the cyclohexane is recycled, it is this process step which accounts for a high proportion of process steam usage.
Therefore, there is a need for alternative separation technology that can reduce steam usage and, as a consequence, reduce the substantial energy cost associated with separation through distillation.
In addition, the process for manufacturing caprolactam requires cyclohexanone as a starting material that is substantially free of cyclohexanol. Currently, this level of purity is achieved commercially through distillation, which again is an energy intensive process.

Liquid-liquid or solvent separation (also known as liquid-liquid or solvent extraction) processes are well known in the art as processes for the separation of components of a mixture. Liquid-liquid separation is based on the transfer of component(s) from one liquid phase into another liquid phase and is used to separate component(s) selectively from a mixture. Mixing two immiscible liquids leads to a phase separation, and the formation of two liquid layers, also known as phases or fractions. The less dense liquid will form the upper layer, and the more dense liquid will form the lower layer. Liquid-liquid separation relies on the different relative solubilities of a component in two immiscible liquids. In particular, if the soluble component is allowed to mix freely with two immiscible liquids, it will partition between the two liquid phases thus formed such that the component will generally be dissolved in one of the liquid phases to a greater extent than in the other liquid phase. Generally, liquid-liquid separation utilises a water-based, or aqueous, phase and an organic phase (comprising an organic solvent) that is substantially immiscible in water. In this instance, when the aqueous phase and the organic phase are mixed with, for example, an aqueous solution of two separable components, if one of the separable components is more soluble in the organic phase it will be separated and become dissolved in the organic phase. Assuming that the other separable component is more soluble in the aqueous phase, then the two separable components will have been separated. Liquid-liquid separation can be a powerful technique provided suitable liquids are used. The traditional aqueous phase/organic phase separation would not be possible for the separation of a cycloalkanol and a cycloalkanone from a cycloalkane because all three of these components would dissolve more readily in the organic phase.
Liquid-liquid extraction technology can also be carried out using two organic phases which are substantially immiscible with each other and in one of which the solubility of the component to be extracted is much greater than the other. A disadvantage of liquid-liquid extraction technology is that final recovery of the components extracted from the initial mixture can be complicated by the volatility of the extracting solvent. Typically final recovery of the components of interest is carried out by distillation, but often the extracting solvent is of comparable volatility to the desired product. Therefore, recovery of the component of interest can be very difficult, and process may also be energy intensive in terms of steam requirements. It is a feature of the present invention that such limitations are avoided by the use of Ionic Liquid(s) as the extracting solvent. Because Ionic Liquids are substantially non-volatile, they do not interfere with the recovery of the component of interest during final recovery. Recovery of said component may, therefore, be effected by simple flash recovery without the need for complicated separation technology with a corresponding reduction in energy (e.g., steam) requirements.
For the manufacture of caprolactam from the oxidation of cyclohexane, it is further necessary to separate cyclohexanol from cyclohexanone, either directly from a mixture of the two or in the presence of cyclohexane. This is because caprolactam manufacture requires only cyclohexanone as a starting material. Conventionally, this separation is carried out by distillation wherein the separation of cyclohexanone from cyclohexanol requires significant energy and a high capital investment in the distillation column required. Liquid-liquid extraction technology is not conventionally used for the separation of cyclohexanone and cyclohexanol since, for this application, it is a requirement of the extraction solvent that it selectively removes one component only from the mixture. Conventional solvents which are suitable for use in a solvent extraction process will not selectively extract cyclohexanol from cyclohexanone.
In liquid-liquid separation, a distribution coefficient for a given separable component can be quoted as a measure of the extent to which a separable component is separated. In traditional liquid-liquid separation, the distribution coefficient is equal to the concentration of the separable component in the organic-based phase divided by the concentration of the separable component in the aqueous phase. The distribution coefficient can be a function of a number of different parameters e.g. temperature.